This invention relates to gas-operated positive displacement pumps.
Gas-operated pumps are known in which gas under pressure moves a wall of a gas chamber against a liquid chamber to squeeze the liquid chamber and thus expel liquid through an outlet. In such gas-operated pumps, the gas is alternatively pressurized and depressurized to squeeze the liquid chamber and expel liquid through a check valve at an outlet port of the pump while the inlet port is blocked and then to permit the liquid chamber to expand so new liquid enters it through an inlet port and inlet check valve.
One prior art pump of this general class is disclosed in U.S. Pat. No. 4,295,801. This patent discloses a pump having an elongated cylindrical body with a reciprocating, gas-driven piston within it to pump liquid from a liquid chamber. In this prior art pump, the piston is driven by expansion of gas chambers and the movement of the piston in pumping liquid switches the pressurized gas from one gas chamber to another to drive the piston in a reciprocating motion.
This prior art type of gas-operated positive displacement pump has a disadvantage in that it is expensive and contains a large number of parts, any one of which may malfunction or clog.
Another prior art type of gas-operated pump of this class is disclosed in U.S. Pat. No. 4,489,779 in which air under pressure is applied directly to the walls of a liquid tube to squeeze it so that it expels fluid. This prior art pump has a central liquid tube which is alternately filled by the hydrostatic head of water surrounding the pump and squeezed to expel water.
This prior art type of pump has a disadvantage in that it only operates in a satisfactory manner when positioned at great depth such as 10 feet so that it has sufficient hydrostatic head to refill the squeezed fluid chamber.
Still another type of prior art pump has been sold by Isco, Inc., 531 Westgate Boulevard, Lincoln, Neb., under the designation, Model 2600 pump. In this pump, air under pressure is alternately applied and removed from a central silicone rubber tube which is circumscribed by a stainless steel cylinder. Water is admitted into a location between the stainless steel cylinder and the expandable silicone rubber so that the alternating pressurizing and relaxing of the silicone rubber pumps liquid out of the pump. Because the silicone rubber is stretched, it snaps back and draws liquid into the liquid compartment, thus enabling it to pump with low hydrostatic pressure surrounding the pump inlet.
The prior art type of pump has a disadvantage in that the elastomeric silicone rubber is not sufficiently inert or chemically unreactive and thus may be damaged by corrosive liquids or hydrocarbon based liquids present in the water being pumped. Also, the silicone rubber may absorb or adsorb some constituents of the water sample thus lowering their concentration in the sample. These constitutents may later desorb which would again give a non-representative concentration of those constituents in the sample taken.